The invention relates to a process for the electrolytic pickling of continuously passing electrically conductive material, in particular metal strip, metal wires or metal profiles, wherein the material successively passes through at least two vessels charged with aqueous electrolyte and wherein the material is subjected to an electric current, as well as an apparatus for carrying out the process.
For the treatment of electrically conductive materials many types of processes are known which employ an electric current, optionally merely for the promotion of the processes. Thus, for example metal strips are electrolytically coated respectively electrolytically pickled. Depending on the manner in which the electric voltage is applied to the strip such processes are sub-divided into two groups, the direct and the indirect processes.
In the direct method the metallic object is directly polarised as a cathode or anode. In large scale pickling plant for the continuous treatment of materials passing therethrough, in particular metal strip, such direct method of current application by means of current take-off rollers, brushes or similar means failed to make the grade because of poor conductivity of the uppermost mill scale layers. Industrial installations have been and still are invariably designed for the application of indirect methods of current application. In this context the metallic strip is passed between pairs of electrodes which alternatingly exhibit opposite polarities. The electric current passes from one electrode by way of the pickling solution to the strip through which it flows preferentially because of the superior conductivity of the metal before being discharged at the next pair of electrodes.
Indirect treatments are for example disclosed in EP-A 93 681 and in EP-A 395 542 which disclose processes and apparatus for the electrolytic coating of elongate metal objects respectively electrically conductive substrates, wherein these work pieces are continuously passed through at least two electrolyte baths. In this context the electrolytes may be the same or different compositions may be employed.
In the respective first bath a cathode is provided and the work piece is therefore anodic, and an anode is provided in the bath intended for coating, such that the work piece is cathodically polarised. The electric circuit is completed by way of the material being treated.
Both aforesaid patent specifications in no way deal with the different problems arising in pickling processes and give no indication as to how the attack by the aggressive ions to which the anode material is subjected, may be counteracted.
Further examples for the electrolytic treatment according to the indirect method are for example the preliminary pickling of super-refined steel in neutral salts, for example sodium sulphate and the subsequent final pickling in mineral acids, for example sulphuric or mixed acids (nitric acid and hydrofluoric acid) such a process is described in AT-PS-252 685.
In AT-PS 39 1 486 a dual stage process for the electrolytic pickling of super-refined steel is described in which in both stages pickling takes place in aqueous neutral salt solutions, alternatingly under anodic and cathodic conditions. In that case electrolyte solutions are also used which contain for example nitrate and fluoride anions, which result in highly aggressive solutions and which therefore subject the anode material to severe attack. This results in comparatively short life expectancies of the anode and accordingly impairs the economics of this process.
Moreover, in many situations no process is known which provides satisfactory results without subsequent mixed acid post-treatment in which the aforesaid anions are likewise present. In all of the aforesaid electrolytic processes the material to be treated is pickled alternatingly anodically and cathodically in the same vessel. This alternating anodic and cathodic treatment takes place also in the regions involving aggressive electrolyte solutions which contain for example fluoride, chloride or nitrate anions, so that in those cases the problem of correct selection of the anode material has not yet been solved economically. Whereas in sulphuric acid solutions respectively neutral electrolytes containing sulphate anions lead anodes have proven themselves, because they are, once being passivied they are subjected to only slight erosion, other anode materials such as for example carbon electrodes or highly alloyed steels as well as carriers coated with more noble metals suffer from the drawback of relatively short life expectancies in conjunction with the aforesaid aggressive ions and of altogether poorer economics due to the increased investment costs. The material which is subjected to anodic polarity is pickled away in the aggressive media and even in the case of coated anodes a dissolution of the coating and resultant rapid erosion of the anode material has been observed in conventional plant, for example in the presence of chloride ions.
Accordingly a need exists to provide a more economic process for the continuous preliminary pickling or complete pickling of electrically conductive materials, in particular of strip metal, metal wires or profiles in which on the one hand in order to improve the treatment effect and to shorten the treatment duration aggressive electrolyte solutions can be employed in conjunction with electric support and which on the other hand, with a view to improved economics, provides long life expectancies of the electrodes, in particular the anodes and low costs as a result of the selection of advantageous anode material.
A need also exists for an apparatus for carrying out such a process.